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Maximum Authorized Altitude (MAA)
MAA represents the highest published altitude or flight level on an airway, within an airspace structure or on direct route segment for which an MEA is designated. Above MAA, reception of navigation aid signals received can originate from far distance stations.

Minimum Crossing Altitude (MCA)
MCA represents the lowest published altitude at which an aircraft must cross a navigational fix when proceeding in the direction of a higher minimum en-route IFR altitude while carrying out a normal climb and staying clear of obstacles. MCA are state prescribed values and are published by the appropriate authorities.

Minimum Cruising Level (MCL)
MCL represents the lowest published level at which an aircraft must cross a navigational fix when proceeding in the direction of a higher minimum en-route IFR altitude while carrying out a normal climb and staying clear of obstacles. MCL are state prescribed values and are published by the appropriate authorities.

Minimum Grid Altitude (MGA)
MGA represents the lowest safe altitude which can be flown off-track.
The MGA is calculated by rounding up the elevation of the highest obstruction within the respective grid area to the next 100ft and adding an increment of
• 1000ft for terrain or obstructions up to 6000ft or
• 2000ft for terrain or obstructions above 6000ft.
e.g. 6345ft obstacle = 6400ft rounded up + 2000ft buffer = 8400ft MGA Shown in hundreds of feet. Lowest indicated MGA is 2000ft.
This value is also provided for terrain and obstacles that would result in an MGA below 2000ft. Exception is over water areas where the MGA can be omitted.

Minimum Sector Altitude (MSA)
MSA represents the safe altitude around a navigation station or aerodrome reference point. If no other information is present, the radius is 25NM and may be valid for a specific sector or approach runway. In case of an RNAV approach, MSA may be replaced by a Terminal Arrival Altitude (TAA) based on one of the procedure fixes. The borders of each sector are defined by bearings in regard to the originating point of the arc. MSAs and TAAs are used for airport navigation and provide a 300m (1000ft) obstacle clearance down to the intermediate approach segment.

Minimum Operating Altitude (MOA)
MOA represents a minimum flight altitude, at which the flight may be planned or operated, taking into account
• minimum standards and operating procedures
• aircraft performance
• current weight
• current weather conditions
Therefore the concerned area is defined through a width beside the route. This value must be determined with each individual flight plan calculation. In general, the MOA is used for en-route and the MORA for off- route.

Minimum Off Route Altitude (MORA)
MORA provides reference point clearance within 10NM of the route centerline (regardless of the route width) and end fixes. The GRID MORA provides reference point clearance within the section outlined by latitude and longitude lines.

Minimum Obstacle Clearance Altitude (MOCA)
The MOCA is the minimum altitude for a defined segment that provides the required obstacle clearance. A MOCA is determined and published for each segment of the route.

Minimum Descent Altitude (MDA) or Minimum Descent Height (MDH)
A specified altitude or height in a non-precision approach or circling approach below which descent must not be made without the required visual reference.
•Minimum descent altitude (MDA) is referenced to mean sea level and minimum descent height (MDH) is referenced to the aerodrome elevation or to the threshold elevation if that is more than 2m (7ft) below the aerodrome elevation. A minimum descent height for a circling approach is referenced to the aerodrome elevation.
•Required visual reference means that section of the visual aids or of the approach area which should have been in view for sufficient time for the pilot to have made an assessment of the aircraft position and rate of change of position, in relation to the desired flight path. In the case of a circling approach, the required visual reference is the runway environment.
•For convenience when both expressions are used, they may be written in the form “minimum descent altitude/height” and abbreviated “MDA/H”.

Decision Altitude/Height (DA/H)
A Decision Height (DH) or Decision Altitude (DA) is a specified height or altitude in the precision approach at which a missed approach must be initiated if the required visual reference to continue the approach has not been acquired.